Great response Lasse - there is also another way
1) Make sure that each Surface has a Boundary and the boundary is set to Sharp and Texture Boundary Property
2) In the areas between the surfaces that get triangulated, you can apply a Null Texture to the surface - that will eliminate all triangles between the surfaces in the combined surface (Texture the surface with "No Texture" - there are other posts on the Forum which address this specific approach). This gives the same result as Lasse's answer to apply a set of boundaries to the surface
Alan